A film resist (also called a dry film resist) for forming a photosensitive layer used as an etching resist in the fabrication of a lead frame or a printed circuit board which are used in mounting a semiconductor integrated circuit, is generally constructed such that a positive or negative photosensitive composition is formed on a support film (a polyester film in many cases) and a protective film is laminated thereon. As photosensitive compositions are known those of an alkali development type in which an unexposed area (negative type) or an exposed area (positive type) is removed with an aqueous alkali solution and those of a solvent development type in which the said area is removed with an organic solvent. The photosensitive composition used is a solid, but is not so hard as its shape does not change, having a certain degree of dimensional deformation.
As an example, a description will be given below of a method of fabricating a printed circuit board or a lead frame in accordance with a metal etching process using an alkali development type film resist.
First, a protective film laminated directly onto a film resist is peeled off and the film resist is laminated to a metallic substrate such as a copper-clad laminate so as to become direct contact with the substrate by means of pressure rolls. Then, a desired pattern is printed to the film resist on the metallic substrate by exposure through a mask with the pattern plotted thereon.
By subsequent development using weak alkali water there is formed a resist pattern on the metallic substrate. The metallic substrate is then subjected to etching with the resist pattern as a mask and thereafter the resist pattern is peeled off using strong alkali water, whereby a printed circuit board or a lead frame is fabricated.
In the process of laminating the film resist to the metallic substrate after removal of the protective film, as pointed out in JP 11-153861A, there occur air voids in the interface between the metallic substrate and the film resist, and due to the presence of such air voids there may occur a pattern loss in forming the resist pattern or there may occur a circuit lead loss in etching the substrate. As mentioned also in the above reference, the thinner the film resist as a photosensitive layer, the more likely the occurrence of such air voids.
In the above reference there is made judgment on the basis of the number of air voids formed, but a more practical method involves forming a plotting pattern with a line width of several ten micrometers (μm) as a substitute for circuit and checking defects on the pattern.
As the material of the protect film, polyesters such as polyethylene terephthalate and polyolefins such as polypropylene and polyethylene are used in many cases, but in Comparative Example 1 described in the above reference a protective film formed of polyethylene is an undesirable example.
The protective film itself in the above reference is required to be improved. More particularly, in the above reference, as a property of the protective film, the number of fish-eyes not smaller than 80 μm is defined. In many cases, such large fish-eyes as are not smaller than 80 μm in diameter result from incorporation of undissolved and deteriorated portions of the material into the film, as noted also in the above reference.
However, in an effort to improve the protective film described in the above reference, even if an attempt is made to fabricate such a film as is small in the number of fish-eyes of even a smaller diameter, say, 30 μm or so, the cause of formation of such fine fish-eyes is not fully clear yet. Besides, even the relation between such fine fish-eyes and the formation of the foregoing air voids is not fully clear yet because of the very small size of the fish-eyes. In more particular terms, the measurement of fish-eyes is based on observation of film transmitted light, and unevenness of the film surface is not determined directly. Therefore, in the case of fish-eyes of a large diameter, there may be a correlation thereof with unevenness of the film surface and the shape thereof, but in the case of fine fish-eyes 30 μm or so in diameter, the fish-eyes merely indicate the trace of resin flow and may not bear a direct relation to unevenness of the film surface. Thus, it is actually difficult to judge whether such fine fish-eyes are correlated or not with unevenness of the film surface and the shape thereof. As a matter of course, it is difficult to judge whether the film permits the formation of the foregoing plotting pattern.
Further, the aforementioned adaptability as a protective film formed of polyethylene is difficult to be changed by changing the film forming method and film forming conditions. It is necessary to alter the essential properties of the polyethylene used. No matter how the film forming method and forming conditions from polyethylene may be changed, there is a limit to the improvement of the aforementioned adaptability as a protective film. By merely changing the film forming method and forming conditions it will be difficult to obtain a useful protective film. For example, even if polyethylene is merely filtered through a filter before or during film formation, no improvement is made in many cases. In the case of the foregoing large fish-eyes not smaller than 80 μm in diameter there is recognized a certain decreasing effect in their number by the adoption of physical removing means such as a filter, and thus it is presumed that there will be an essential difference from fine fish-eyes 30 μm or so in diameter.